One part of the research being proposed is aimed at elucidation of the chemical mechanisms involved in the transition of fibrinogen to fibrin. We will first try to identify the covalent structures (A and B) in the NH2-terminal part of fibrinogen which are activated by the release of fibrinopeptides. We will also try to identify the complementary structures (a and b) which are located in more COOH-terminal regions of fibrinogen. The interaction of these sites (A:a and b:B) in activated fibrinogen will be studied by a variety of techniques including light-scattering. We will also study to what extent the interaction of these sites may be regulated by specific hydrolysis by plasmin of critical bonds in the fibrinogen molecule. The second part of our proposal deals with interaction of platelets with different physical forms of fibrinogen, e.g., fibrinogen adsorbed to artificial surfaces and fibrinogen conjugated to Sepharose. We will try to identify the structures in fibrinogen and the platelets which participate in the interaction. We will also study the interaction between ADP-exosed platelets and fibrinogen. We will attempt to identify the structures in both fibrinogen and platelets which participate in the interaction.